1. Field of the Invention
This invention relates to a phase change optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information. The phase change optical recording media are of great interest since the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of phase change type used chalcogenide systems such as Ge--Sb--Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state.
When information is recorded in the optical recording medium of phase change type, the laser beam applied is of high power (recording power) that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallizing temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallizing temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single light beam.
In the optical recording medium of phase change type, dielectric layers are generally formed on opposite sides of the recording layer. Requirements for the dielectric layers are:
(1) the dielectric layers should be capable of protecting the recording layer and the substrate from heat histerisis as a result of the laser beam irradiation; PA1 (2) the dielectric layers should be capable of amplifying the reproduced signal by making use of optical interference effect of the lights reflected from boundaries between the layers; and PA1 (3) the recording and erasing properties can be regulated by adjusting thermal conductivity and the like of each dielectric layer. PA1 a) alteration in the thickness of the recording layer by the migration of the material constituting the recording layer in the direction of the recording track, PA1 b) alteration in the composition of the recording layer due to element diffusion between the recording layer and the adjacent dielectric layers, PA1 c) deformation or damage of the dielectric layer, and PA1 d) exfoliation at the interface between the recording layer and the dielectric layer, or the dielectric layer and the reflective layer. PA1 the first dielectric layer comprises a dielectric layer 1a on the side of the substrate and a dielectric layer 1b on the side of the recording layer, and PA1 the dielectric layer 1a contains zinc sulfide and silicon oxide as its main components, and PA1 the dielectric layer 1b contains silicon nitride and/or germanium nitride; silicon oxide; zinc sulfide and silicon oxide; or chromium oxide as its main component(s); and PA1 when the dielectric layer 1b contains zinc sulfide and silicon oxide as its main components, the dielectric layer 1b has silicon oxide content [SiO.sub.2 /(ZnS+SiO.sub.2)] of at least 40 mol %, the silicon oxide content being determined by calculating the zinc sulfide and the silicon oxide in terms of ZnS and Sio.sub.2, respectively; and PA1 the second dielectric layer contains zinc sulfide and silicon oxide; silicon oxide; oxide of a rare earth metal; or germanium nitride as its main component(s). PA1 the first dielectric layer comprises a dielectric layer 1a on the side of the substrate and a dielectric layer 1b on the side of the recording layer, and PA1 the dielectric layer 1a contains zinc sulfide and silicon oxide as its main components, and PA1 the dielectric layer 1b contains silicon nitride and/or germanium nitride; silicon oxide; zinc sulfide and silicon oxide; or chromium oxide as its main component(s); and PA1 when the dielectric layer 1b contains zinc sulfide and silicon oxide as its main components, the dielectric layer 1b has silicon oxide content [SiO.sub.2 /(ZnS+SiO.sub.2)] of at least 40 mol %, the silicon oxide content being determined by calculating the zinc sulfide and the silicon oxide in terms of ZnS and SiO.sub.2, respectively; and PA1 the second dielectric layer comprises a dielectric layer 2a on the side of the recording layer and a dielectric layer 2b on the side of the reflective layer, and PA1 the dielectric layer 2a contains zinc sulfide and silicon oxide as its main components, and the dielectric layer 2b contains silicon oxide or oxide of a rare earth metal as its main component, or PA1 the dielectric layer 2a contains silicon nitride and/or germanium nitride; or chromium oxide as its main component(s), and the dielectric layer 2b contains a dielectric material having a thermal conductivity lower than the dielectric layer 2a as its main component. PA1 said recording layer has a light absorption coefficient at recording/reproducing wavelength such that: EQU Ac/Aa.gtoreq.0.9 PA1 the first dielectric layer comprises a dielectric layer 1a on the side of the substrate and a dielectric layer 1b on the side of the recording layer, and PA1 the dielectric layer 1b contains silicon nitride and/or germanium nitride as its main component(s), and PA1 the dielectric layer 1a contains zinc sulfide and silicon oxide as its main components; and PA1 at least the region of the second dielectric layer in contact with the recording layer contains germanium nitride and/or silicon nitride; oxide of a rare earth metal; silicon oxide; or zinc sulfide and silicon oxide as its main component(s); and when such region contains zinc sulfide and silicon oxide as its main components, the region has silicon oxide content [SiO.sub.2 /(ZnS+SiO.sub.2)] of at least 40 mol %, the silicon oxide content being determined by calculating the zinc sulfide and the silicon oxide in terms of ZnS and SiO.sub.2, respectively.
Typical dielectric layers which meet such requirements are those containing highly refractive ZnS as their main component. For example, Japanese Patent Application Kokai (JP-A) No. 103453/1988 discloses an optical information recording material having a dielectric layer containing a mixture of ZnS and SiO.sub.2. The merits described therein include increase in sensitivity for the power of incident light upon recording, and increase in the number of erasing/overwriting operations of the dielectric material. The increase in the sensitivity is said to have been realized by optimizing thermal constant of the dielectric layer, and the increase in the number of erasing/overwriting operations is said to have been realized by reducing the laser power required for the recording and the erasure. JP-A 103453/1988 discloses that SiO.sub.2 /(ZnS+SiO.sub.2) is preferably in the range of 10 to 30 mol % since the laser energy required for the recording and the erasure is minimum when SiO.sub.2 /(ZnS+SiO.sub.2) is in such range.
However, in the optical recording medium of phase change type having the recording layer comprising Ge--Sb--Te based material or the like, decrease in the erasability and increase in the jitter, and hence, decrease in C/N occurs with the repeated overwriting operations, and the medium becomes unoverwritable after approximately several thousand times. Major causes for such decrease in the C/N with the repeated overwriting operations include:
Prevention of the deformation and the damage of the dielectric layer is suggested in, for example, in JP-A 64937/1990. JP-A 64937/1990 describes provision of a heat-resistant protective layer on one surface or both surfaces of the recording layer, and a protective layer having a bulk modulus smaller than that of the heat-resistant protective layer on at least one surface of the heat resistant protective layer, and the heat-resistant protective layer is thereby prevented from deformation and rupture and adaptability to repeated recording operations is improved. The exemplary materials disclosed therein for constituting the protective layer of smaller bulk modulus are MoS.sub.2, ZnS, ZnSe and the like, and the exemplary materials disclosed therein for constituting the heat-resistant protective layer are silicon carbide, silicon nitride, and aluminum oxide. In Example 2 of JP-A 64937/1990, SiN.sub.x layers of 20 nm thick are formed on opposite sides of the recording layer, and a ZnS layer of 100 nm is formed on the SiN.sub.x layer on the side of the laser beam incidence and a ZnS layer of 200 nm is formed on the SiN.sub.x layer on the other side. There is described that the medium of such constitution exhibited good adaptability to repeated recording operations of several ten thousand times. It is believed that the element diffusion from the protective layer comprising ZnS to the recording layer is prevented in this example by the SiN.sub.x layer.
However, when the inventors of the present invention evaluated the medium of the layer constitution as described in the example of JP-A 64937/1990, the number of overwritable operations was only about 10,000.
Prevention of the migration and segregation of the recording layer is suggested, for example, in JP-A 127176/1996. JP-A 127176/1996 describes addition to phase change type recording film of Ge--Sb--Te system of at least one element X selected from the group consisting of Cr. Ag, Ba, Co, Ni, Pt, Si, Sr, Au, Cd, Cu, Li, Mo, Mn, Zn, Al, Fe, Pb, Na, Cs, Ga, Pd, Bi, Sn, Ti, V, In and lantanoid elements to promote precipitation of the high melting components having a melting point higher than that of the phase change components to thereby prevent the migration and segregation of the recording layer during the recording and the erasure. There is described that repeated overwriting operations in excess of 10.sup.5 times is enabled by such constitution. In the examples of JP-A 127176/1996, Cr, Cu, Cr+Tl, Cr+N, or Cr+Se is used for the element X, and the element X is added in an amount of at least 3 at %, and initialization is conducted for several hundred times in order to promote the precipitation of the high melting metal phase for the purpose of preventing the migration and the segregation.
The initialization which should be conducted for several hundred times as described in JP-A 127176/1996 invites increase of the production cost, and is undesirable.
It should be noted that JP-A 127176/1996 discloses an example of Ag addition. Ag, however, is not added to the recording film, but to a mask layer provided for the purpose of super resolution reproduction.